Why they put a wood inside concrete well filled with water in a cemetry? I was visiting a cemetery in Hamburg (Ohlsdorf Cemetery) and I noticed something strange. Around the cemetery, there are a few water wells made by concrete. There is water inside for people to water flowers on a grave. In each well, there is a wooden cross floating on the water. I thought first that there have only symbolic meanings but my friend told me that those wooden crosses were put to prevent the wells being cracked in winter due to freezing in winter. It doesn't make sense to me. Is there any physical explanations of this?
 A: Yes, there is.  If the well freezes, then there will be a disk of ice on the surface, which can be quite thick.  This ice can exert potentially a very large force on the concrete of the well: which can in some cases be large enough to cause it to fail (this is how freeze-thaw weathering of rock works, for instance).
If you insert some object with a relatively low elastic modulus into this disk, then it will deform (compress) instead: it will be, really, a buffer, or a spring.
The final part of the trick is that you want to make sure that this object is always on the surface where the ice will form, even if the surface moves as the well fills & empties.  Well, the answer to that is to make it float on the surface, and that's exactly what these things are doing.  Wood is a good choice because it will become waterlogged and thus will float rather low in the water, which means it will keep being useful for even a fairly thick disk of ice.  It's also, of course, historically a very widely-available material which, conveniently, floats.
A cross shape is good because it means the object can't float off to one side of the water where it will stop being useful, but also leaves some big gaps so you can actually get at the water (a large disk, for instance, would work as well for preventing the problem but would make the well useless).
This trick will fail if the well freezes all the way through, as ice will then form below the wood, but that is presumably a rare enough occurrence that it's not a problem. 
